Polycrystalline rhombohedral boron nitride and method of producing the same

Abstract

PCT No. PCT/JP86/00095 Sec. 371 Date Jul. 3, 1986 Sec. 102(e) Date Jul. 3, 1986 PCT Filed Feb. 27, 1986 PCT Pub. No. WO86/05169 PCT Pub. Date Sep. 12, 1986.The specification discloses a polycrystalline boron nitride of high purity and high density consisting essentially of rhombohedral crystals in which the three-fold rotation axes, parallel to the c-axis in the notation of hexagonal crystal system, of the crystals have a preferred orientation. The polycrystalline rhombohedral boron nitride can be obtained as bulk or thin film articles with desired shapes by chemical vapor deposition including the steps of introducing a source gas of boron and a source gas into a reactor containing a heated substrate and depositing boron nitride onto the heated substrate, wherein a diffusion layer of the source gas of nitrogen and/or the carrier gas is formed around the substrate. The polycrystalline rhombohedral boron nitride such obtained is very useful in applications such as crucibles for melting semiconductors, various jigs for high temperature services, high-frequency insulator, microwave transmission window and source material of boron for semiconductor. Further, the boron nitride is also ideal as a starting material for high pressure phase cubic boron nitride.

We claim: 
     
       1. A polycrystalline rhombohedral boron nitride of a bulk density ranging from 1.90 to 2.24 g/cm 3  and obtained as a self supporting massive material or as a thin film, consisting essentially of rhombohedral crystals in which the 3-fold rotation axes (parallel to the c-axis in the notation of the hexagonal crystal system) of said crystals have a preferred orientation. 
     
     
       2. The polycrystalline rhombohedral boron nitride as claimed in claim 1, wherein said boron nitride includes, in part, boron nitride which has a turbostratic or amorphous structure. 
     
     
       3. The polycrystalline rhombohedral boron nitride of claim 1, wherein the rhombohedral boron nitride has the X-ray diffraction patterns of FIGS. 2B and 2C when taken perpendicular to the 3-fold rotation axes of the rhombohedral crystal and when taken parallel to the 3-fold rotation axes of the rhombohedral crystal. 
     
     
       4. A polycrystalline rhombohedral boron nitride having a bulk density ranging from 1.90 to 2.24 g/cm 3  prepared by a process consisting essentially of: passing a source gas of boron and a source gas of nitrogen each through a gas inlet tube, with or without the presence of a diluting carrier gas, into a reactor containing a heated substrate in a fashion such that a diffusion layer of said source gas of nitrogen and/or said carrier gas is formed over the substrate heated to a temperature lower than 1700° C. under conditions specified by the relationship: FR/(SxP 2  ×L)≧2, wherein S is the cross-sectional area (cm 2 ) of the end of said gas inlet tube for introducing said nitrogen source gas or said carrier gas, FR is the flow rate (ml/min) under the normal conditions of 20° C., 1 atm of said nitrogen source gas and/or said carrier gas, P is the total gas pressure (Torr) within said reactor, and L is the distance (cm) between the end of said gas inlet tube and the deposition surface of said substrate, said source gas of boron being a boron halide, a boron hydride, a nitrogen-containing boron compound or an alkyl boron compound and said source gas of nitrogen being a nitrogen hydride, an ammonium halide or molecular nitrogen; and   depositing polycrystalline boron nitride as rhombohedral crystals onto said heated substrate such that the 3-fold rotation axes (parallel to the c axis in the notation of the hexagonal crystal system) of said crystals have a preferred orientation.   
     
     
       5. In a method of producing boron nitride by chemical vapor deposition which includes the steps of introducing a source gas of boron and source gas of nitrogen through a gas inlet tube, with or without a diluting carrier gas, into a reactor containing a heated substrate and depositing boron nitride onto said substrate, the improvement which comprises: forming a diffusion layer consisting essentially of said source gas of nitrogen plus any carrier gas for the gases introduced into the reactor around the substrate heated to a temperature lower than 1700° C. under conditions specified by the relationship: FR/S×P 2  ×L)≧2, wherein S is the cross-sectional area (cm 2 ) of the end of said gas inlet tube for introducing said nitrogen source gas or said carrier gas, FR is the flow rate (ml/min) under the normal conditions of 20° C., 1 atm of said nitrogen source gas and/or said carrier gas, P is the total gas pressure (Torr) within said reactor, and L is the distance (cm) between the end of said gas inlet tube and the deposition surface of said substrate and introducing a source gas of boron separate from said nitrogen source gas, thereby depositing polycrystalline boron nitride of a bulk density ranging from 1.90 to 2.24 g/cm 3  consisting essentially of rhombohedral crystals onto said heated substrate in which the 3-fold rotation axes (parallel to the c axis in the notation of the hexagonal crystal system) of said crystals have a preferred orientation.   
     
     
       6. The method as claimed in claim 5, in which the temperature of said heated substrate is in the range of 1200° to less than 1700° C. 
     
     
       7. The method as claimed in claim 6, in which the temperature of said heated substrate is in the range of 1450° to 1650° C. 
     
     
       8. The method as claimed in claim 5, in which the total gas pressure in said reactor is below 20 Torr. 
     
     
       9. The method as claimed in claim 8, in which the total gas pressure in said reactor is within the range of 1 to 5 Torr. 
     
     
       10. The method as claimed in claim 5, 6, 7, 8 or 9, wherein said source gas of boron and said source gas of nitrogen are boron trichloride and ammonia respectively.